It is well recognized in the railway industry that, either through manufacturing processes or through natural environmental processes and normal use, rails of a railroad track develop certain detrimental flaws. The flaws may include both non-critical and critical defects, for example, transverse defects, vertical shear or split head defects, horizontal shear or split head defects, rail end batter, or crushed head defects.
Safe operation on a rail may continue as long as the flaws remain noncritical. However, in time, even non-critical flaws may abscess or degrade into critical defects, and new flaws will arise. If the flaws are left unattended, the resulting defects could lead to a range of problems including catastrophic failure and train derailment. Catastrophic or even lesser failures present a financial, health and safety risk to the railway industry, transported goods and personnel, as well as surrounding homes and businesses. Such failures can be prevented, or at least decreased in frequency through routine inspection and maintenance. Further, routine maintenance can be made more cost-effective through selective repair, which is in turn facilitated by flaw detection.
To detect rail flaws or defects, ultrasonic testing has been employed. Vehicles and rail car-mounted inspection apparatus have been built to travel along the track and continuously perform ultrasonic inspection of the rails of the railroad track in situ. For example, carriage wheel type ultrasonic inspection apparatus, such as those of U.S. Pat. No. 7,850,748 or such as the Sperry® 1900 model inspection apparatus, are capable of detecting and measuring several types of microscale (internal) rail defects or flaws. However, other means are needed for detecting macroscale (external) defects such as crushed heads or localized surface collapses.